1. Field of the Invention
The present invention relates to a device for verifying the calibration of sensors and, more particularly, to an orbiting device for verifying the calibration of infrared sensors.
2. Description of the Related Art
Infrared sensors are designed to detect and measure thermal radiation emanating from remote objects. In order to accurately determine the infrared characteristics of remote objects, the values of the calibration coefficients used with an infrared sensor must be developed. Developing the values of such coefficients involves exposing the sensor to primary reference standards having precisely known infrared characteristics at known frequencies, measuring the sensor's output, and developing coefficients such that the compensated sensor output is as close as possible to the value of the primary source.
The accuracy of the calibration coefficients associated with the sensor may be subsequently verified by measuring the sensor's output when it is exposed to secondary reference standards having infrared characteristics that are known with less precision. The term “infrared sensor” as used herein includes the object on which the infrared energy is focused (e.g., focal plane array or scanning array), the associated electronics that create a digital output from the array's analog output, and the software used to transform the relative output of the pixels in the sensor, on a pixel by pixel basis, into a common absolute reference by applying one or more calibration coefficients to the measured output of each pixel.
In one example, a space-based infrared sensor is calibrated on the ground using one or more known temperature sources (primary reference standards). The accuracy of the coefficients used to calibrate the sensor, as well as the stability of the sensor and associated electronics, are subsequently verified on-orbit by having the sensor view a star with a known radiant intensity and temperature (a secondary reference standard), and measuring the compensated sensor's output against the known value of the star's radiant intensity.
As used herein, “verifying the calibration of an infrared sensor” means comparing the compensated output of a sensor that is viewing an object having a known infrared intensity at frequency bands of interest, to the object's known infrared intensity at the same frequency bands, and determining whether or not the sensor system is operating correctly and within specification, and/or potentially updating the calibration coefficients used with the sensor system to reduce any observed errors in the compensated output of the sensor.
Both active and passive devices have been used as secondary reference standards. Active devices use powered energy sources, such as electrical heaters, to heat a radiant surface to a desired reference temperature. Temperature sensors are used to ensure that the radiant surface is maintained at the reference temperature
Passive devices are heated by ambient temperatures and external radiant energy sources, such as the sun. These devices are provided with surface coatings having a selected absorptivity and emissivity to maintain the radiant surface at a desired reference temperature. Such passive devices are typically designed to act as black body sources, i.e., sources that emit radiation that is entirely dependent upon their temperatures.
Conventional devices have several drawbacks. For example, U.S. Pat. No. 5,716,030 discloses a spacecraft telescope door having a panel for providing a radiant temperature reference for verifying a calibration of an infrared sensor. The panel is coated with a highly emissive paint having a low reflectance. The panel is also provided with electrical heaters and temperature sensors to allow either active or passive operation.
One drawback of this device is that the telescope door must be closed when the panel is in use. In addition, where active mode operation is required, the useful life of the device is limited by the longevity of its power supply.
A further drawback of this device is the limited dynamic range over which the calibration can be verified using the device. For example, when the telescope door closes and the panel is presented to the infrared sensor, all of the pixels of the sensor are exposed to the panel at the same time and at the same temperature, as the image of the door is not in focus on the focal plane array.